Increased emphasis in recent years on the energy efficiency of industrial processes has led to efforts to partially recover the enormous expenditures of energy required in paint finishing operations on automotive production lines. Such energy recovery efforts are coupled with increasingly stringent requirements to reduce the pollutants produced by such industrial processes.
One aspect of the paint finishing operation which consumes a particularly heavy share of the total energy required is the paint curing oven, wherein the painted surfaces are cured after application of the paint by being passed through ovens having radiator surfaces heated by the use of gas or other fuel-fired heaters.
Traditionally, the products of combustion in a fuel-fired heater would be individually vented or exhausted through the atmosphere passing upwardly and through the roof of the factory. Many modern large automotive manufacturing facilities having a large number of paint curing oven heaters space them along the length of the curing oven. The individual venting of each heater represents a considerable expense since the roof must be penetrated for each vent and flashing and other hardware installed. The resultant installations are also expensive to maintain.
The shop area around the ovens must also be ventilated since the radiation of heat from the ovens is considerable, and also some leakage of fumes is inevitable, thus requiring additional roof vents.
In addition, if any filtering or other treatment of the exhaust gas is required, the number of individual treatment installations would render the treatment of the exhaust gases to be impractical or inordinately expensive. If the recovery of the heat energy in the exhaust gases were attempted by collection of the gases into a common ductwork, the length of such installation, as well as the relatively high temperature of these exhaust gases, i.e., 300.degree.-750.degree. F., would entail considerable capital expense. This is in part due to the need to provide a costly and complex ducting system, since such ductwork should be insulated to minimize the substantial heat loss which would otherwise occur due to the relatively elevated temperature of the exhaust gas. Also, the considerable thermal contraction and expansion of the ducting undergoing such wide temperature variations indirectly receiving such hot gases requires the use of elaborate thermal expansion compensating connections.
Finally, the need to maintain the proper back pressure in each exhaust stack, while compensating for changes in flow which would occur by shutting off individual heaters, creates the need for costly flow balancing dampers in the duct system.
There are many other lower grade heat sources in industrial installations from which additional heat energy could be potentially recovered. However, the potential energy may be insufficient to justify the capital expenditure. For example, the air above the paint curing ovens is generally heated by radiation to relatively modest temperature levels, i.e., of 95.degree. F. As another example, cascade body coolers are commonly employed in which air is successively recirculated in stages over the vehicle car bodies as they pass through the cooler. The air is then exhausted to the atmosphere at a temperature well above ambient levels, but much cooler than the heater exhaust gases.
Accordingly, it is an object of the present invention to provide a relatively low cost ducting system for the recovery of heat contained in relatively high temperature gases such as exhaust gases from fuel-fired heaters such as are in paint curing ovens.
It is another object of the present invention to provide such a ducting system which also provides for the simultaneous recovery of heat energy from relatively lower grade heat sources.
It is a further object of the present invention to provide such a heat recovery system for a paint curing oven which also acts to prevent leakage from the oven interior and thereby reduces the ventilation requirements for installations within a shop building.